Vehicles having an electric motor in addition to an engine as a power source, such as hybrid car, plug-in hybrid car, hybrid vehicle and hybrid electric vehicle, and transport machinery (hereinafter referred to as vehicle) have been put into practical use. In addition, electric vehicles having no engine and driven only by an electric motor have also been put into practical use. A battery, such as a lithium-ion battery which is compact and has a large capacity, has been used as a power source for driving the electric motor for such vehicle. For vehicle application, the battery may be of a battery pack which is composed of plural cell blocks each having plural battery cells connected in series. The series connection of the battery cells allows supply of the voltage that is required to drive an electric motor of a vehicle and the series or parallel connection of the cell blocks allows supply of electrical current of the required capacity and also provides a higher voltage.
The property of a lithium-ion battery is varied greatly depending on the temperature, and the remaining capacity and charging efficiency of the battery are influenced greatly by the temperature in the environment where the battery is used. This is all the more true of the environment where the vehicle is used.
As a result, there occurs a great variation in the remaining capacity and the output voltage of the respective battery cells in the cell block. In this case, when the voltage of any one of the battery cells is decreased below a threshold, power supply for the whole of the battery needs to be stopped or suppressed, with the result that the power efficiency is lowered. Thus, battery equalization control for equalizing the voltages of the battery cells needs to be performed, and equalization of the voltages of the cell blocks also needs to be performed.
One of the known battery equalization method is a passive equalization that causes an overcharged or overdischarged battery cell to discharge through a bypass resistor connected in parallel to the battery cell so as to equalize the voltages of the battery cells. Another known battery equalization is an active equalization that takes charge from a battery cell of a higher voltage and delivers the charge to a battery cell of a lower battery cell through a converter circuit using an inductor or transformer. In either battery equalization, discharging or charging is done until the voltage of such battery cell reaches target voltage. In the active equalization, the target voltage corresponds to the average of the highest and lowest voltages of the plural battery cells. The battery cell with the highest voltage is discharged to the target voltage, while the battery cell with the lowest voltage is charged to the target voltage. In the passive equalization, the target voltage corresponds to the lowest voltage of the plural battery cells. The battery cells with higher voltages are discharged to the target voltage.
The battery cells have different internal resistances depending on the variation of the characteristics and the degree of deterioration of the respective battery cells. When the equalization is done so that the voltage of the battery cell reaches the target voltage, the voltage of the battery cell monitored during discharging or charging varies from the actual output voltage of the battery cell due to the internal resistance of the battery cell. Thus, when the equalization is done so that the monitored voltage of the battery cell reaches the target voltage, the final voltage of the battery cell is not the same due to the internal resistance of the battery cell. Correction of such variation requires repeated equalization and is time consuming, resulting in increased battery energy loss. In the active equalization, current for equalization is small when the monitored voltage is close to the target voltage, resulting in decreased efficiency and increased loss of the equalization circuit.
The present invention is directed to providing an apparatus and a method which allows quick and efficient equalization of the voltages of the plural battery cells having different internal resistances thereby to offer optimum equalization control.